The ever increasing demand for esthetic dental restorations, both by patients and dentists, has stimulated the improvement of resin composites and, nowadays, this material responds for the vast majority of direct, chair- side treatments delivered each year. However, the stress that inherently develops from the polymerization of the organic matrix still poses a challenge to the bonded interface between the tooth and the restoration, which reduces the life-time and reliability of such treatments. This study proposes a relatively simple approach that will tackle polymerization stress while increasing conversion and toughness with no changes in the operatory technique. Thiourethane oligomeric additives will be designed to be incorporated into the resin matrix with the objective of delaying the onset of gelation, which will provide extended opportunity for viscous flow, ultimately leading to increased conversion and significantly reduced stress development. The additive will also provide more homogeneous networks, increased toughness and greater refractive index for better match with the inorganic fraction, in turn improving depth of cure. Two aims are proposed: 1) Thiourethane oligomeric species will be synthesized, comprising tethered thiols into their backbones. Thiol and isocyanate starting materials will be judiciously chosen to produce a wide array of backbone flexibilities. Analog oligomers based on thiolenes and urethanes will be used as controls, as well as to probe the mechanism of toughening by thiourethanes. Methacrylate network structure and stress development will be examined as a function of thiourethane flexibility, additive concentration and secondary monomer matrix (including monomers commonly used in dental restorative materials). Mechanical properties in flexure, degree of conversion and reaction kinetics will be used as screening tools to identify the oligomer providing the best compromise between delayed gelation and increased conversion/mechanical properties (especially toughness). In selected cases, polymerization stress will be evaluated concomitantly with conversion, providing a direct demonstration of the extent of polymerization at which the majority of stress (onset of vitrification) starts to develop. 2) Since the organic portion corresponds to only 30-40 % of the total composite volume, the best materials identified under aim 1 will also be tested as filled compositions to ensure that the improvements obtained for unfilled resins hold for composites. This aim will also include the synthesis of a new silane coupling agent containing thiourethane bonds and tethered thiols. This will take advantage of the large surface area of the fillers as toughening and chain-transfer sites to amplify the stress reduction and the gain in mechanical properties. Due to their inherently higher refractive index, thiourethane oligomers will improve the optical clarity of the final composite, increase light transmission through the material and lead to augmented depth of cure. The expected outcome of this project is to substantially reduce polymerization shrinkage stress, while increasing conversion and mechanical properties, ultimately overcoming the major drawbacks of current direct polymeric restoratives.